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Document number: N3747

Date: 2013-09-01

A Universal Model for Asynchronous Operations

by Christopher Kohlhoff

Excerpt:

... In this paper, we will first examine how futures can be a poor choice as a fundamental
building block of asynchronous operations in C++. The extended std::future brings
inherent costs that penalise programs, particularly in domains where C++ use is prevalent
due to its low overheads. An asynchronous model based on a pure callback approach, on the
other hand, allows efficient composition of asynchronous operations.

However, cognizant that some C++ programmers may have valid reasons for preferring a
futures-based approach, this paper introduces a universal model for asynchronous
operations. This model supports both lightweight callbacks and futures, allowing the
application programmer to select an approach based on appropriate trade-offs.

Finally, we will see how this universal model can be leveraged to support other models of
composition. This paper presents implementation experience of the universal model, which
includes several pure library implementations of resumable functions, or coroutines.
Programmers have the opportunity to express asynchronous control flow in an imperative
manner, without requiring the addition of new keywords to the language.